Tidal flows, oceanic currents and water current in rivers represent considerable amounts of unexploited energy. Numerous attempts have been made to provide efficient and reliable systems for extracting energy from such flowing liquid.
Tidal mills, such as structures resembling a wind mill and arranged on the seabed, are known. Such arrangements typically have the disadvantage that considerable forces and bending moments arise, which must be withstood or absorbed by a supporting tower structure.
WO-2004/067957 describes a helical turbine device that may be disposed under water. The shaft of the screw turbine is connected to a generator. The screw turbine and the generator arrangement are rotatably connected to a foundation on the seabed. The turbine blade has sufficient buoyancy as to raise the screw turbine to a diagonal position in the water when in use.
An arrangement of the type mentioned in the introduction has been disclosed in WO-2009/093909. This publication describes a helical turbine arrangement including two helical turbines with opposite pitches, arranged in parallel and overlapping each other. The screw turbine shafts are connected to generators. The arrangement is rotatably connected to a bottom mount on the seabed. The turbine blades have sufficient buoyancy as to raise the screw turbine to a diagonal position in the liquid when in use. This prior art technology makes it possible to convert energy of flowing liquid, such as tidal flows, to electric energy, without the disadvantage of the bending moments/forces of the prior, rigid tidal mills. However, it still has certain drawbacks with regard to overall performance, such as energy efficiency and stability of power output. In particular, tests have shown that the power output of the generators suffer from excessive fluctuations or variations.